imgcodecs(webp): multiple fixes

- don't reallocate passed 'img' (test fixed - must use IMREAD_UNCHANGED / IMREAD_ANYCOLOR)
- avoid memory DDOS
- avoid reading of whole file during header processing
- avoid data access after allocated buffer during header processing (missing checks)
- use WebPFree() to free allocated buffers (libwebp >= 0.5.0)
- drop unused & undefined `.close()` method
- added checks for channels >= 5 in encoder

modules/imgcodecs/src/loadsave.cpp

@@ -707,11 +707,22 @@ static bool imwrite_( const String& filename, const std::vector<Mat>& img_vec,
 
     encoder->setDestination( filename );
     CV_Assert(params.size() <= CV_IO_MAX_IMAGE_PARAMS*2);
-    bool code;
-    if (!isMultiImg)
-        code = encoder->write( write_vec[0], params );
-    else
-        code = encoder->writemulti( write_vec, params ); //to be implemented
+    bool code = false;
+    try
+    {
+        if (!isMultiImg)
+            code = encoder->write( write_vec[0], params );
+        else
+            code = encoder->writemulti( write_vec, params ); //to be implemented
+    }
+    catch (const cv::Exception& e)
+    {
+        std::cerr << "imwrite_('" << filename << "'): can't write data: " << e.what() << std::endl << std::flush;
+    }
+    catch (...)
+    {
+        std::cerr << "imwrite_('" << filename << "'): can't write data: unknown exception" << std::endl << std::flush;
+    }
 
     //    CV_Assert( code );
     return code;